1. Field of the Invention
The present invention relates to the field of lasers, and in particular to lasers capable of delivering light of a single mode at high power and through a large aperture.
2. Description of Prior Art
The field of optics and optical circuits is maturing rapidly and efforts to miniaturize the individual optical elements are underway. Among the most important of these elements are lasers for delivering high-quality monochromatic light. It is believed that vertical-cavity surface-emitting lasers (VCSELs) and related laser diodes may satisfy many of the requirements imposed on the new generation of light sources. To be effective a VCSEL should have a large aperture, provide single-mode emission, have a low threshold current, high power, and exhibit a repeatable and predetermined polarization orientation.
There are two generic VCSEL structures. The first is a laser-post-type index-guided laser described in detail by J. L. Jewell et al. in Electronics Letters, 25, 1989, p. 1123 and by R. S. Geels et al. in IEEE Photonics Technology Letters, Vol. 2, No. 4., p. 234, April 1990. The laser post or laser cavity has an active region made of a laser medium, usually GaAs, InGaAs, InGaAsP, or InP. The laser medium can be present in bulk form or as quantum wells, quantum wires, or even quantum dots. A set of reflectors or mirrors, usually multilayer Bragg reflectors, are positioned on top and bottom of the active region. Frequently, spacers are placed between the active region and the reflectors to set the cavity length. This structure is typically grown on a substrate and the finished post is encapsulated by a current-blocking layer, usually polyimide or photoresist, which has a significantly lower index of refraction than the materials making up the laser post. By virtue of its lower refractive index the current-blocking layer traps the light generated by the laser medium inside the post (i.e., the blocking layer ensures that the post becomes a highly efficient waveguide). The resulting laser produces a single longitudinal mode but multiple transverse modes at low threshold currents.
Unfortunately, while providing suitable quality transverse and longitudinal mode laser light with a high degree of intrinsic polarization, this type of laser is incapable of sustaining single-mode emission even at low power levels. That is because the driving current induces higher order transverse and longitudinal lasing modes. Consequently, the laser-post-type index-guided laser lacks the necessary performance criteria for application in optical circuits.
The second type of VCSELs are proton-implanted gain-guided lasers described by the applicant in IEEE Photonics J. Quantum Electronics, 27, p. 1402, 1991 and in IEEE J. Quantum Electronics, 27, p. 1377, 1991. As a result of their large size, commonly about 10 .mu.m, these lasers have a high threshold current. In addition, they maintain linear polarization of the basic transverse mode (TEM.sub.00) near threshold current only, i.e., at currents equal to 1.4 to 2 times the threshold current. At higher currents the polarization directions become random when more transverse modes begin lasing. Moreover, the polarization varies in a stochastic manner from one laser to another. Thus, gain-guided lasers can not be set up to operate in concert, e.g., in array structures, and have a severely limited operating range.
The above solutions clearly indicate that the properties requisite of a laser source for optical applications can not be presently combined in one laser structure. In particular, prior art devices specifically fail to produce a single, fundamental mode (TEM.sub.00) laser operable over a wide power range and exhibiting a predetermined polarization. Also, prior art lasers are limited to small apertures typically ranging from 2 to 5 .mu.m. Meanwhile, optimal aperture dimensions are much larger, e.g., between 8 .mu.m and 30 .mu.m, depending on the application.